A large portion of the United States periodically suffers from earthquakes, tornados, or hurricanes. Low-level wooden buildings, including virtually all residential structures, are particularly susceptible to damage from these events. Consequently, even one such event can damage or destroy large numbers of wood-framed structures and their contents, causing billions of dollars of damage, displacing thousands of people from their homes, and seriously injuring or killing their occupants.
Earthquakes, tornados, and hurricanes destroy low-level wood-framed structures in two primary ways: creating high shear forces in the walls and uplifting the structure from its foundation. Lateral forces created by wind pressure or by seismic activity create substantial shear forces in the walls of the building which it would not normally experience. Further, the walls of a wood-framed building are generally weakest against shear loads. Consequently, violent shear forces can tear a standard wood-framed building apart. Uplifting of the building from its foundation also results from the abnormal atmospheric pressures and wind forces associated with tornados and hurricanes, and from the seismic motion of the ground during an earthquake.
Because of the significant damage and loss of life than can result from a tornado, hurricane, or earthquake, the Uniform Building Code (UBC) began to impose requirements in the 1970s for providing additional shear strength in the walls of low-level wood-framed structures. Originally, plywood shear panels nailed onto a wooden wall frame and attached to the building's base with hold-downs were used to provide the extra shear strength needed to meet the UBC requirements.
Plywood shear panels have several disadvantages. They take up a great amount of space and restrict the height to width ratio and design flexibility of buildings. This problem occurs because the plywood shear panels must be a certain size in order to comply with the strict strength requirements of the UBC. Additionally, the end vertical studs to which the plywood shear walls attach must be bulky 3.times.5 or 4.times.4 studs instead of the customary 2.times.4 studs in order to accommodate the nailing schedule used to attach the plywood shear wall to the skeletal frame. Builders using plywood shear panels must follow a complex nailing schedule and utilize a specific type of nail to meet those requirements. A large amount of time and skilled labor is required to hammer in all of the nails that are required by the prior art, adding to construction time and expense. In addition, significant inspection time is required to ensure that the proper nailing schedule and nail type were used, adding to construction time and placing a burden on city building inspectors.
Hold-downs were used along with plywood shear panels to provide the necessary shear strength and address the problem of uplifting. Two primary types of hold-down were used. The first consisted of a bolt that attached the plywood shear wall to a bottom plate, which is then attached to the foundation. L-shaped braces were also used to attach the end vertical studs to the bottom plates; those braces were then attached in turn to the foundation. Neither of these methods directly attaches the shear wall to the foundation. Rather, a bottom plate intermediates between the two, creating a failure point. As the structure ages, a wooden bottom plate may deteriorate for several reasons. The constant pressure of the structure on the wooden bottom plate for year after year can crush or compress it. Insects such as termites can attack and destroy the wooden bottom plate. As the wood dries out, it can shrink or become brittle. Consequently, as the wooden bottom plate ages and deteriorates, the hold-down nut remains stationary on the hold-down bolt, forming a gap between the nut and the wooden bottom plate. Such a loosened hold-down loses much of its effectiveness for uplift resistance. Further, because these hold-downs were not attached in line with the uplift forces, they were subject to significant moment forces during uplift, creating extra strain on the hold-downs and increasing the likelihood of failure.